A production terminal can include a permanently moored vessel for storing and/or processing hydrocarbons from undersea wells for later transfer to a tanker. One type of mooring apparatus for such a terminal is the SALM (single anchor leg mooring) type which includes an anchor leg having a lower end anchored to the sea floor and an upper end which lies near or above the sea surface and is connected by a yoke or the like to the vessel. Conventional SALM terminals have often utilized a buoy near the top of the anchor leg to assure that the leg is always under tension, so that a long slender anchor leg can be utilized which can withstand large tension forces but which would buckle under a low compression load. However, the large buoy has several disadvantages, including sensitivity to waves and currents and added expense.
One approach to eliminating the need for a large buoy on the anchor leg is to utilize a device mounted on the dedicated vessel to apply a constant upward force to the top of the anchor leg. U.S. Pat. No. 4,088,089 by Flory shows several systems of this type wherein various devices on the vessel are employed to apply tension to the anchor leg. Such devices on the vessel include a counter weight which moves within a shaft formed in the vessel and which is connected to a cable to the anchor leg, a vertical piston on the vessel which is connected by a cable to the anchor leg, a winch on the vessel, and a counter weight mounted on a yoke lying over the vessel deck and with the counter weight lying even higher above the deck. While all of these approaches can apply tension to the anchor leg, they have serious disadvantages. The application of forces through cables involves the use of several vulnerable moving parts that decrease reliability. The use of a counter weight high above the vessel deck subjects the system to instabilities, due to the great height of the counter weight, which is necessary to permit considerable downward pivoting by it, and also requires isolated deck region on the vessel to accommodate the counter weight when it moves downwardly. A mooring system of the type which utilized an apparatus near the sea surface other than a buoy on the vessel to apply upward forces on a SALM that anchors a dedicated vessel, but which was of simple and reliable construction, would be of considerable value.